Physics Of The Curve Ball
Geege Schuman stashed this in Baseball
From F*ck Yeah Fluid Dynamics:
Because the ball is spinning forward, pressure forces are unequal between the top and bottom of the ball, with the bottom part of the baseball experiencing lower pressure. As with a wing in flight, this pressure difference between surfaces creates a force — for the curveball, downward.
This is known as the Magnus effect — as the ball is in motion through the air, the air produces lift perpendicular to the spin axis.
As the ball rotates around the ball's horizontal axis, it creates additional pressure on the top of the ball, which makes it drop faster than gravity alone. If it was spinning in a different way, for instance around the vertical axis, the pressure would push it right or left.